This invention concerns systems and methods for measuring the position and distance of a remote object.
Several methods have been employed to make non-contact measurement of distance or relative displacement of an object from a transducer. One class of such methods relies on optical principles. In measuring distance optically, parallax, wave interference, or time of flight are often employed. Parallax-based optical measurement can be further subdivided into multi-view (e.g., stereography) or structured light source (e.g., laser triangulation) systems. Most system designs seek to optimize one or more operational characteristics versus cost. Transducer size, range, operating distance, repeatability, accuracy, resolution, sample frequency, operating temperature range, sensitivity to environment, and shadowing effects are typical characteristics. There is a need for a low cost, low-to medium-range distance measurement system that exhibits exceptional repeatability, accuracy, speed, and low sensitivity to temperature and ambient light.
In general terms, one aspect of the invention is a system for measuring distance to a target along a Z axis within a measurement range. The system comprises at least one (preferably three) optical signal generator(s) that are aimed to project onto the target a signal in the form of a two dimensional geometric figure (as opposed to a line, pairs of non-intersecting lines, or a single point). The signal generator(s) are typically but not necessarily aimed directly at the target, i.e., along the Z-axis. Regardless of the direction of aiming, there could be an intermediate optical element, such as a mirror or lens, along the path; however, such elements are not required and in some embodiments they would not be preferred because they complicate the operation of the system (although the techniques to account for the use of such optical elements are well within the skill of the art). An image device is aimed (typically but not necessarily directly at the target) to capture a reflected image of the two dimensional geometric figure. Again, there could be an intermediate optical element, such as a mirror or lens, along the path. An image analyzer is connected to the image device. The image analyzer computes distance to the target from a geometric parameter of the reflected image. Typical geometric parameters are size, shape, area, orientation, length of perimeter (circumference), and other equivalent parameters.
A similar but alternative aspect of the invention is a system for measuring location of a target in three dimensions within a measurement range. This system is like the one just described, but the image analyzer also locates a point on the target relative to a field of view of the image device. Generally, but not necessarily, the point on the target is the geometric center of a known contrasting feature of the target. The system then uses the distance to the target and the point it has located, along with a predetermined magnification curve for the image device, to compute the location of the target in X and Y directions.
Another alternative aspect of the invention is a method for measuring distance to a target along a Z axis within a measurement range. The method does not require use of the equipment specifically described in the system aspects set forth above. The method comprises: projecting onto the target a two dimensional geometric figure; capturing a reflected image of the geometric figure; and computing distance to the target from a geometric parameter of the reflected image.
Yet another alternative aspect of the invention is a method for measuring location of a target in three dimensions within a measurement range. The method does not require use of the equipment specifically described in the system aspects set forth above. The method comprises: projecting onto the target a two dimensional geometric figure; capturing a reflected image of the geometric figure; computing, from a geometric parameter of the reflected image, the distance to the target along a Z axis; locating a point on the target relative to a field of view; and using the distance it has computed, the point it has located, and a predetermined magnification curve for the image device, to compute the location of the target in X and Y directions.
In any of the aspects of the invention described above, it is preferred but not required for the center of projection of the two dimensional geometric figure to lie along the Z axis. The two dimensional geometric figure may be a whole or partial polygon (the most preferred being a triangle), circle, or other figure having symmetric portions. The two-dimensional geometric figure need not be a closed figure so long as enough of the overall figure is visible to construct and analyze the image.
Typically, the target is retroreflective and/or mounted to an object within the measurement range, but neither of these is required.
Suitable image devices include electronic cameras of any design. Suitable signal generators are laser line generators and optical line projectors.
One specifically preferred technique is for the image analyzer to use a calibrated magnification of the system and the geometric parameter to determine the distance to the target. Another is for the image analyzer to use a calibrated lookup table to determine the distance to the target. Yet another is for the image analyzer to use a mathematical equation to determine the distance to the target.
Finally, another aspect of the invention is a target for determining location within a measurement range. This is the preferred, but not required, target for use in the aspects of the invention described above. The target comprises a base which has a flat retroreflective surface region. Within, and flush with, the flat retroreflective surface lies a flat contrasting inset region. A transparent protective cover (e.g., polycarbonate) lies over the retroreflective and contrasting regions. The preferred but not required shape of the target is circular. Another preferred but not required feature is some means of attachment to another object. Any means of attachment is suitable, and all such means are equivalents of each other for purposes of this invention.